I think like everyone, I go through the same journey whenever I find out about a new technology..

Huh? –> This is really cool –> I use it everywhere –> Hmm, sometimes it’s not so great

Remember when people were writing websites with XSLT transforms? Yes, exactly. XML is great for storing a data structure as a string, but you really don’t want to be coding your application’s business logic with it.

However there are some scenarios where ORMs are best avoided. Let me introduce my easy to use, ‘when to use an ORM’ chart.

It’s got two axis, ‘Model Complexity’ and ‘Throughput’. The X-axis, Model Complexity, describes the complexity of your domain model; how many entities you have and how complex their relationships are. ORMs excel at mapping complex models between your domain and your database. If you have this kind of model, using an ORM can significantly speed up and simplify your development time and you’d be a fool not to use one.

The problem with ORMs is that they are a leaky abstraction. You can’t really use them and not understand how they are communicating with your relational model. The mapping can be complex and you have to have a good grasp of both your relational database, how it responds to SQL requests, and how your ORM comes to generate both the relational schema and the SQL that talks to it. Thinking of ORMs as a way to avoid getting to grips with SQL, tables, and indexes will only lead to pain and suffering. Their benefit is that they automate the grunt work and save you the boring task of writing all that tedious CRUD column to property mapping code.

The Y-axis in the chart, Throughput, describes the transactional throughput of your system. At very high levels, hundreds of transactions per second, you need hard-core DBA foo to get out of the deadlocked hell where you will inevitably find yourself. When you need this kind of scalability you can’t treat your ORM as anything other than a very leaky abstraction. You will have to tweak both the schema and the SQL it generates. At very high levels you’ll need Ayende level NHibernate skills to avoid grinding to a halt.

If you have a simple model, but very high throughput, experience tells me that an ORM is more trouble than it’s worth. You’ll end up spending so much time fine tuning your relational model and your SQL that it simply acts as an unwanted obfuscation layer. In fact, at the top end of scalability you should question the choice of a relational ACID model entirely and consider an eventually-consistent event based architecture.

Similarly, if your model is simple and you don’t have high throughput, you might be better off using a simple data mapper like SimpleData.

So, to sum up, ORMs are great, but think twice before using one where you have a simple model and high throughput.